Power amplifiers for transmission which are included in wireless communication devices consume much electric power among other components in the wireless communication devices. Therefore, improving the power efficiency of power amplifiers is an important task to be accomplished in the development of wireless communication devices. In recent years, the wireless communication standards have seen mainstream efforts directed to an amplitude modulation scheme for improving the spectral efficiency. According to the amplitude modulation scheme, since strict requirements are imposed on signal distortions, power amplifiers need to be operated in a high backoff (low input power) state for better linearity. However, if a power amplifier is operated in the high backoff state, then the power amplifier has its power efficiency lowered. Recently, EER (Envelope Elimination and Restoration) has been intensively researched as a technology for increasing the power efficiency of power amplifiers and keeping linearity between input and output signals.
The EER technology is a scheme for amplifying highly efficiently an input signal (modulated signal) including an amplitude-modulated (AM) component and a phase-modulated (PM) component. Specifically, only the PM component that is left by removing the AM component from the input signal is amplified, and the amplified PM component is amplitude-modulated with the removed AM component, thereby linearly amplifying the input signal and restoring the original waveform thereof. FIG. 1 shows a configuration of a power amplifier according to the background art which is based on the EER technology.
FIG. 1 is a block diagram showing the configuration of the power amplifier according to the background art which is based on the EER technology.
As shown in FIG. 1, the power amplifier according to the background art which is based on the EER technology comprises signal generating circuit 147, RF (Radio Frequency) amplifier 109, pulse modulator 104, driver amplifier 116, switching amplifier 105, low-pass filter 106, and bandpass filter 107.
Signal generating circuit 147 extracts an AM component included in an input signal, and outputs the extracted AM component as amplitude component signal 111 through terminal 145 to pulse modulator 104. Signal generating circuit 147 also extracts a PM component included in the input signal, and outputs the extracted PM component as phase component signal 112 through terminal 146 to RF amplifier 109.
Pulse modulator 104 pulse-modulates amplitude component signal 111 to generate a rectangular-wave signal, and outputs the rectangular-wave signal to driver amplifier 116.
According to the rectangular-wave signal output from pulse modulator 104, driver amplifier 116 drives switching amplifier 105 to amplify the rectangular-wave signal efficiently. The amplified rectangular-wave signal is smoothed by low-pass filter 106, and then supplied through terminal 142 to RF amplifier 109.
RF amplifier 109 comprises transistor 101, input power supply circuit 108, and output power supply circuit 140. RF amplifier 109 amplifies phase component signal 112 output from signal generating circuit 147. An output signal from RF amplifier 109 is amplitude-modulated with the smoothed rectangular-wave signal supplied from switching amplifier 105 through low-pass filter 106 and terminal 142, i.e., amplified amplitude component signal 114.
Input power supply circuit 108 that is connected to the gate of transistor 101 is usually supplied with a constant DC voltage from a power supply device, not shown, through terminal 141.
The signal amplified by RF amplifier 109 (output signal 115) is processed by bandpass filter 107 to remove unwanted band components therefrom, and then supplied through terminal 144 to an antenna device, not shown, or the like.
FIG. 2 is a block diagram showing a configurational example of the signal generating circuit shown in FIG. 1, and FIG. 3 is a block diagram showing another configurational example of the signal generating circuit shown in FIG. 1. Signal generating circuit 147 shown in FIG. 2 is of a configuration optimum for an application wherein an RF signal is input to input terminal 143 of the power amplifier, and signal generating circuit 147 shown in FIG. 3 is of a configuration that is optimum for an application wherein a baseband signal is input to input terminal 143 of the power amplifier.
Signal generating circuit 147 shown in FIG. 2 comprises amplitude detector 103 for extracting an AM component from an RF signal as an input signal and outputting the extracted AM component as amplitude component signal 111, and limiter 102 for removing the AM component of the input signal. Amplitude detector 103 extracts the AM component of the input signal (RF signal) input from terminal 143, and outputs the extracted AM component as amplitude component signal 111 from terminal 145. Limiter 102 removes the AM component of the input signal (RF signal) input from terminal 143, and outputs phase component signal 112, which represents a remaining PM component, from terminal 146. Signal generating circuit 147 shown in FIG. 2 also includes delay corrector 153 which is capable of adjusting the delay time difference between amplitude component signal 111 and phase component signal 112.
Signal generating circuit 147 shown in FIG. 3 comprises baseband signal processing circuit 150 and VCO 151. Baseband signal processing circuit 150 should preferably comprise a DSP (Digital Signal Processor) and a D/A (digital-to-analog) converter. Baseband signal processing circuit 150 outputs amplitude component signal 111, which represents the AM component of the baseband signal as the input signal, to terminal 145, and also outputs a phase component signal, which represents the PM component of the baseband signal, to VCO 151. In baseband signal processing circuit 150, the DSP calculates and extracts the AM component of the baseband signal input from terminal 143 according to a digital processing process, and the D/A converter converts the AM component into an analog signal and thereafter outputs the analog signal as amplitude component signal 111 from terminal 145. Furthermore, the DSP calculates and extracts the PM component of the baseband signal input from terminal 143 according to a digital processing process, and the D/A converter converts the PM component into an analog signal and thereafter outputs the analog signal as a phase component signal from terminal 145. Baseband signal processing circuit 150 controls VCO 151 with the same phase component signal.
VCO 151 is controlled by the phase component signal from baseband signal processing circuit 150 to output a phase component signal which has been up-converted into an RF signal.
With the power amplifier shown in FIG. 1, signal generating circuit 147 outputs phase component signal 112 with sufficiently large electric power to keep transistor 101 of RF amplifier 109 saturated state in operation at all times. The drain of transistor 101 of RF amplifier 109 is supplied with amplitude component signal 114 through terminal 142 and output power supply circuit 140 to amplitude-modulate phase component signal 112 amplified by transistor 101 with amplitude component signal 114. Therefore, the power amplifier can amplify the input signal with high power efficiency and maintain linearity between the input and output signals.
On the other hand, ET (Envelope Tracking) is known as another technology for increasing the power efficiency of power amplifiers and keeping linearity between input and output signals.
The ET technology is a scheme for amplifying an input signal including an AM component and a PM component, extracting the AM component of the input signal, and amplitude-modulating the amplified signal with the extracted AM component for thereby increasing the power efficiency and keeping linearity between input and output signals. FIG. 4 shows the configuration of a power amplifier according to the background art which is based on the ET technology.
FIG. 4 is a block diagram showing the configuration of the power amplifier according to the background art which is based on the ET technology.
As shown in FIG. 4, the power amplifier according to the background art which is based on the ET technology is different from the power amplifier based on the EER technology shown in FIG. 1 as to configurational and operational details of signal generating circuit 148. The configurational and operational details of the other components are the same as those of the power amplifier based on the EER technology shown in FIG. 1 and will not be described below. In FIG. 4, components other than signal generating circuit 148 are denoted by the same reference characters as those of the power amplifier shown in FIG. 1.
Signal generating circuit 148 extracts an AM component included in an input signal, and outputs the extracted AM component as amplitude component signal 111 through terminal 145 to pulse modulator 104. Signal generating circuit 148 outputs modulated signal 149 that is proportional to the amplitude of the input signal which includes the AM component and a PM component, through terminal 146 to RF amplifier 109.
FIG. 5 is a block diagram showing a configurational example of the signal generating circuit shown in FIG. 4, and FIG. 3 is a block diagram showing another configurational example of the signal generating circuit shown in FIG. 4. Signal generating circuit 148 shown in FIG. 5 is of a configuration that is optimum for an application wherein an RF signal is input to input terminal 143 of the power amplifier, and the signal generating circuit shown in FIG. 6 is of a configuration that is optimum for an application wherein a baseband signal is input to input terminal 143 of the power amplifier.
Signal generating circuit 148 shown in FIG. 5 comprises amplitude detector 103 for extracting an AM component from an RF signal as an input signal and for outputting the extracted AM component as amplitude component signal 111. Amplitude detector 103 extracts the AM component of the input signal (RF signal) input from terminal 143, and outputs the extracted AM component as amplitude component signal 111 from terminal 145. The input signal input from terminal 143 is supplied to amplitude detector 103 and is also output as modulated signal 149 from terminal 146. Signal generating circuit 148 shown in FIG. 5 also includes delay corrector 153 which is capable of adjusting the delay time difference between amplitude component signal 111 and modulated signal 149.
Signal generating circuit 148 shown in FIG. 6 comprises baseband signal processing circuit 150 and quadrature modulator 152. In baseband signal processing circuit 150, the DSP calculates and extracts the AM component of the baseband signal input from terminal 143 according to a digital processing process, and the D/A converter converts the AM component into an analog signal and thereafter outputs the analog signal as amplitude component signal 111 from terminal 145. Furthermore, the D/A converter converts the input baseband signal into an analog signal and thereafter outputs the analog signal to quadrature modulator 152.
Quadrature modulator 152 up-converts the baseband signal output from baseband signal processing circuit 150 into an RF frequency signal, and outputs the RF frequency signal as modulated signal 149 from terminal 146.
With the power amplifier shown in FIG. 4, signal generating circuit 148 outputs modulated signal 149 with sufficiently large electric power to keep transistor 101 of RF amplifier 109 saturated state in operation at all times, thereby enabling RF amplifier 109 to have the function of limiter 102 shown in FIG. 2. Specifically, the power amplifiers based on the EER technology and the ET technology operate according to common principles except that the PM component of the input signal is input to RF amplifier 109 according to the EER technology and modulated signal 149 including the AM component and the PM component is input to RF amplifier 109 according to the ET technology. Therefore, the power amplifier based on the ET technology can also amplify the input signal with high power efficiency and maintain linearity between the input and output signals.
If the power amplifier shown in FIG. 1 or FIG. 4 is employed to power-amplify an RF signal having a bandwidth of several MHz such as those used in the W-CDMA (Wideband Code Division Multiple Access) communication process, then driver amplifier 116 and switching amplifier 105 need to perform a switching operation in a frequency range from several tens to several hundreds MHz. Furthermore, if the EER technology and the ET technology are applied to power amplifiers provided in radio base stations, then it is necessary for switching amplifier 105 to output a high voltage of several tens of volts. According to the present device and circuit technologies, however, driver amplifier 116 and switching amplifier 105 that operate under a high voltage of several tens of volts have a switching rate that is limited to about several hundreds kHz at maximum. A scheme for avoiding such an operational limitation on driver amplifier 116 and switching amplifier 105 has been proposed by JP-A No. 2005-244950, for example.
FIG. 7 is a block diagram showing another configurational example of the power amplifier according to the background art which is based on the EER technology. The power amplifier shown in FIG. 7 is illustrated in FIG. 24 of JP-A No. 2005-244950.
Data generator 301 shown in FIG. 7 outputs an amplitude component signal and a phase component signal of a transmission signal. The phase component signal is added to an RF signal by angle modulator 303 and output to amplitude modulator 305. The amplitude component signal is decomposed into a low-frequency amplitude component signal and a high-frequency amplitude component signal by frequency discriminator 302. Amplitude modulator 305 amplitude-modulates the phase component signal with a high-frequency amplitude component signal generated by high-frequency voltage controller 304, and amplitude modulator 307 amplitude-modulates the phase component signal with a low-frequency amplitude component signal generated by low-frequency voltage controller 306. According to this arrangement, high-frequency voltage controller 304 may have relatively small output power though the operating frequency thereof is high, and low-frequency voltage controller 306 may have a relatively low operating frequency though the output power thereof is large. Therefore, high-frequency voltage controller 304 and low-frequency voltage controller 306 shown in FIG. 7 do not need to have both high-voltage output and fast switching operation, but may be realized by the present device and circuit technologies.
However, the above power amplifiers according to the background art are problematic in that the power efficiency of RF amplifier 109 shown in FIGS. 1 and 4 is lowered when the voltage (power supply voltage) supplied to output power supply circuit 140 of RF amplifier 109 is lowered. FIG. 8 shows power efficiency characteristics when the RF amplifier 109 shown in FIG. 1 is supplied with a constant power supply voltage (when it is in conventional operation), and also shows power efficiency characteristics when the RF amplifier 109 shown in FIG. 1 is in EER operation.
As shown in FIG. 8, RF amplifier 109 has its power efficiency made better when it is in EER operation than when it is in conventional operation. However, even when RF amplifier 109 is in EER operation, the power efficiency thereof is low at the time that the output power thereof is small. The reduction in the power efficiency of RF amplifier 109 at the time that the output power thereof is small is responsible to a reduction in the average power efficiency of the overall power amplifying circuit.
The power amplifiers according to the background art are also disadvantageous in that the power efficiency of switching amplifier 105 shown in FIGS. 1 and 4 is lowered when the output voltage (average voltage) of switching amplifiers 105 is lowered. As with the reduction in the power efficiency of RF amplifier 109 described above, the reduction in the power efficiency of switching amplifier 105 is responsible for a reduction in the average power efficiency of the overall power amplifying circuit.
Specifically, in the case where the amplitude component of the input signal has a large dynamic range and the output power of the power amplifier is small, the power efficiency of the RF amplifier and the switching amplifier of the power amplifiers according to the background art based on the EER technology and the ET technology is lowered and cannot be sufficiently improved.
Furthermore, if the power amplifiers according to the background art shown in FIGS. 1 and 4 are employed to power-amplify an RF signal having a wide bandwidth, then driver amplifier 116 and switching amplifier 105 needs to have both high-voltage output and a fast switching operation. However, such requirements cannot be met by the present device technologies. Accordingly, the power amplifiers shown in FIGS. 1 and 4 have a limited range of applications.
The power amplifier shown in FIG. 7 does not require driver amplifier 116 and switching amplifier 105 to have both high-voltage output and fast switching operation. However, the arrangement has a problem in that a signal representative of the input signal which is highly accurately restored cannot be produced as the output signal.
According to the arrangement shown in FIG. 7, when amplitude modulator 307 is saturated state in operation, the output amplitude of amplitude modulator 307 is nearly independent of the output amplitude of amplitude modulator 305. Therefore, the output signal of amplitude modulator 307 does not reflect the amplitude of the high-frequency amplitude component signal generated by high-frequency voltage controller 304. Conversely, when amplitude modulator 307 is linearly operated, the output amplitude of amplitude modulator 307 is virtually unchanged by the low-frequency amplitude component signal generated by low-frequency voltage controller 306. Therefore, the output signal of amplitude modulator 307 does not reflect the amplitude of the low-frequency amplitude component signal generated by low-frequency voltage controller 306. Consequently, since the output signal reflects only either the amplitude component signals generated by high-frequency voltage controller 304 or low-frequency voltage controller 306, it is difficult to produce a signal representative of the input signal which is highly accurately restored, as the output signal.